The term "composite transducer" as used herein denotes a transducer which includes regions of an electrically active material (i.e. a piezoelectric material) which are embedded in a matrix of a second material. Preferentially, the second material is an electrically passive material (i.e. an insulator). The second material may be elastometric or rigid and may have acoustic properties which are considerably different from the acoustic properties of the active material.
Composite piezoelectric transducers, and methods for their production, are described, for example, in Composite Piezoelectric Transducers; R. E. Newnham et al; Materials in Engineering, Vol. 2, December 1980, pages 93-106, which is incorporated herein, by reference, as background material.
In one prior art method for manufacturing composite transducers, piezoelectric material is extruded to form a continuous strand. The strand is then cut to form piezoelectric rods of a desired length. Parallel, spaced-apart rods are assembled in a fixture and the spaces between the rods are filled with resin (see, for example, U.S. Pat. No. 4,122,725). This method of assembling rods in a fixture is extremely tedious and is impractical when the center-to-center spacing of the rods is very small (for example on the order of 50 microns or less). Alignment fixtures having these fine dimensions are expensive and difficult to handle. Moreover, strands of piezoelectric material often take on unwanted curvatures which prevent accurate assembly in a fixture.
A method for fabricating transducers is also disclosed in German Patent No. 3124561. A block of piezoelectric material is grooved with two series of cuts, the second series lying at a 90.degree. angle to the first series, to form a matrix of active regions. Passive material is then introduced into the grooves between the active regions and electrodes are attached to the individual elements.
German Patent No. 3019478 teaches a method for fabricating ultrasound transducer combs wherein the height of active transducer elements is substantially greater than their width. Strip-shaped plates of piezoelectric material are mounted in a stack, separated by spacers. A passive filler material is introduced betweeen the stacked plates. Several cuts are made in the stack, along a plane perpendicular to its width, to provide a transducer comb in which the transducer elements are parallel to one another. This method is not suitable for producing regions of active material having widths and heights on the order of 50 microns or less and does not yield active regions which are isolated from one another, on all sides, by passive material.